Explosion-proof electric heater is a closed loop heat exchanger (HE) partially filled with a heat transfer fluid in which an electrical element is immersed. The explosion-proof heaters are designed to be used in hazardous environments, where an explosive mixture of petroleum gases or vapors, or flammable dust (for example but not limited to: aluminum/magnesium dust, carbon black, coal, coke, flour, starch, grain dusts) may exist under normal operating conditions.
FIG. 1 illustrates an exploded view of an exemplary explosion-proof heater 100. The explosion-proof heater 100 includes a closed loop heat exchanger 102 which is initially kept under vacuum. The electrical element heats up a glycol-based heat transfer fluid and produces steam, which travels upwards through a number of heat exchange columns 104. Collectively, the exchange columns 104 may be considered as the tube portion of the closed loop heat exchanger 102. The heat exchange columns 104 generally have a carbon steel core and aluminum fins. The closed loop heat exchanger 102 also have a can portion 110 which contains a liquid phase of the heat transfer fluid. The can portion 110 is made of carbon steel. An external fan 106, installed on an explosion-proof motor 108, blows ambient air onto the heat-exchange columns 104 causing steam to condense. The generated heat is forcibly convected into the environment.
Referring to FIG. 2, the closed loop heat exchanger 102 contains a pressure relieve valve (PRV), preferably at the top of the a closed loop heat exchanger 102. The pressure relieve valve comprises a PRV body 202, a plug 204, a gasket 206, a gasket holder 208, a spring 210, a locking nut 212 and an adjustment knob 214. The pressure relieve valve releases the heat transfer fluid when the closed loop heat exchanger malfunctions. The malfunction of the heat exchanger may be caused by inadequate cooling. Generation of steam, constant heating and lack of cooling will dramatically increase pressure inside the heat exchanger 102.
Commercially available heat transfer fluids generally contain ethylene or propylene glycol solutions in a concentration of about 10% or more, and a corrosion inhibitor, for example, di-potassium phosphate (K2HPO4) in a concentration of about 2% or more. In general, ethylene glycol is toxic and not environment friendly. Propylene glycol is less toxic compared to ethylene glycol, it nevertheless pollutes the environment.
During the long term operation of the heat exchanger 102, a deposit inside the pressure relieve valve was observed. The deposit mainly contained K2HPO4 upon analysis. In certain occasions, the deposit would totally obstruct the pressure relieve valve. A totally plugged PRV and malfunctioning unit, as described above, would lead to a rupture of the heat exchanger or the entire explosion-proof electric heater. As the heaters are located in hazardous locations, any of the above events are highly undesirable.
When lower K2HPO4 concentration was used, it may not provide adequate corrosion protection without glycol. However, the presence of glycol, e.g. propylene glycol in exchange heater does not add any benefit to exchange heater start-up or its performance during regular operation. The propylene glycol is a flammable fluid. The 10% propylene glycol in water may not pose any fire hazard, however, in case of leakage in the heat exchanger 102, the heat exchanger 102 may lose steam and may cause concentration of propylene glycol in the heat exchanger 102 to increase, which may cause a potential fire hazard.
The presence of heater elements may also decompose glycol into organic acids or formaldehyde, resulting in excessive generation of gas which in turn may cause a high pressure that will engage the pressure relieve valve.
In addition, the K2HPO4 based inhibitor dissolves only in liquid-phase and does not protect the vapor phase of the heat exchanger.
US patent application 2012/0061611 describes a heat transfer fluid comprising water, glycerol from 30% to 60% (w/w), and a surfactant. The surfactant is added to reduce the viscosity of the fluid caused by the high concentration of glycol. The surfactant has further characteristics for corrosion inhibition, a high thermal capacity and helps to protect the fluid from degradation at high temperatures.
US patent application 2006/0163528 describes an aqueous antifreeze composition comprising 10 to 50% by weight of one or more dicarboxylic acids, thus providing protection against corrosion.
Therefore, there is a need of a glycol-free heat exchange fluid. There is further a need to a glycol-free heat exchange fluid whereby only a small amount of corrosion inhibitor is able to protect the internals of the heat exchanger. There is further a need to a glycol-free heat exchange fluid as it protects both the liquid phase and the vapor phase of the heat exchanger (heater).